This appears to be a fragment, not a sentence; and it's not clear which word you are asking about -- is it the first "probability" or the second "probabilities" you are considering?
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Andrew Leach♦Jun 6 '12 at 23:35

Since a probability vector is a nonnegative vector whose coordinates sum to 1 ... If that is what you are talking about "a probability vector" would do it. It's much more a maths question .

edit: following your edit, it's more clear (it initially looked like your v was standing for versus), then, since in a probability vector there are several probabilities, the plural is ok for the second occurrence.

edit #2: I guess you are talking about being in one given vertex, which is associated to a vector giving the probabilities for going to one of the other vertices, right ?